Hot workability of austenitic stainless steel alloys

ABSTRACT

Improving the hot workability of austenitic stainless steel alloys by first providing a cast slug with a controlled microstructure, usually by inoculation of the molten material with a metal such as titanium to produce a generally equiaxed structure, followed by forging of the cast slug into the desired shape.

ilnited States Patent Inventors Delmar 0. Stanley;

John F. Kocis, both of Mentor, Ohio 691,187

Dec. 18, 1967 Sept. 21, 1971 TRW lnc.

Cleveland, Ohio Appl, No. Filed Patented Assignee 110T WORKABILITY OFAUSTENITIC STAINLESS STEEL ALLOYS [56] References Cited UNlTED STATESPATENTS 3,385,739 5/1968 Danis 148/12.1 1,251,341 12/1917 Churchward..75/130.5 X 2,197,955 4/1940 Valenta 75/126 2,380,821 7/1945 Breeler....148/135 X 2,738,267 3/1956 Pakkala... 148/135 X 2,806,782 9/1957 Faber75/58 X Primary Examiner-L. Dewayne Rutledge Assistant Examiner-JosephE. Legru Att0rneyHill, Sherman, Meroni, Gross and Simpson ABSTRACT:Improving the hot workability of austenitic stainless steel alloys byfirst providing a cast slug with a controlled microstructure, usually byinoculation of the molten material with a metal such as titanium toproduce a generally equiaxed structure, followed by forging of the castslug into the desired shape.

HOT WORKABILITY F AUSTENITIC STAINLESS STEEL ALLOYS BACKGROUND OF THEINVENTION 1. Field ofthc Invention This invention deals with the art ofimproving the ductility of austcnitie stainless steel alloys bycontrolling the grain size and the secondary dendrite arm spacing in acast slug which is to be forged, specifically by adding small,controlled amounts of titanium which serve to provide titaniumcarbonitride compounds in the casting and thereby provide sites for thecrystallization of equiaxed grains.

2. Description of the Prior Art The fabrication of exhaust valves forinternal combustion engines usually involves one of two methods. Thefirst is that of extruding and coining the valve from a mill wroughtvalve steel. The second involves casting the valve directly. Theadvantage of the wrought mill steel for forging processes is that thealloy exhibits good forgeability as a result of the wrought slugductility. The ductility of the alloy is partially attributed to thebreaking down of the cast structure from the ingot stage of millprocessing. The disadvantage of this material is the added costnecessary to convert from the ingot stage to bar stock. The cast-typematerial, on the other hand, can be poured directly into an exhaustvalve form and eliminates the cost of working the alloy to bar stock.However, the cast material is more susceptible to defects from castingand is generally much lower in tensile ductility. The material does,however, exhibit good high temperature properties because of its caststructure. This property is desirable in an exhaust valve that isoperating at elevated temperatures.

More recently, the desirable properties of the cast and wroughtstructures have been combined through a process which involves castingthe slug from which the forging is to be made, so that some of the eaststructure is still retained, particularly in the head portion of thefinished valve. This type of technique, however, has been found topresent the difficulty of hot tearing, particularly in the region of theouter diameter of the head. The present invention substantially reducesthis tendency and provides the advantages of a partially cast, partiallywrought structure by providing a slug which has improved forgingproperties, particularly improved ductility at forging temperatures.

SUMMARY OF THE INVENTION The present invention relates to improvedforged articles and the method of making the same wherein during thecasting of the slug which is to be forged, the microstructure of theaustenitic stainless steel alloy is refined to provide a casting havinga grain size of from 50 to 400 grains per square inch and a secondarydendrite arm spacing of from 0.02 to 0.06 mm. Experiments have shownthat this type of microstructure lends itself particularly well toforging, and substantially reduces the tendency of the forged article toexhibit hot tearing.

Although other methods of grain refinement can be used to achieve thepurpose of the present invention, we particularly prefer to inoculatethe melt with titanium metal, in amounts of from 0.05 to 0.25 percent byweight. Such inoculation results in the distribution of titaniumcarbonitride compounds throughout the solidified mass, these compoundsserving as sites for initiating crystallization of equiaxed grains ofthe metal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To start with, we shouldmention that the present invention is applicable to any austeniticstainless steel which is otherwise suitable for use in exhaust valvesfor internal combustion engines. The improved results of the invention,however, are most noticeable when the carbon content is not in excess ofabout 0.90 percent but is greater than about 0.40 percent, i.e., withmedium carbon stainless steel. The grain refinement takes place equallywell with higher carbon steels (or more than 0.90 percent carbon) butthe improvement in ductility is not as pronounced as it is in the caseof the medium carbon steels.

The following analyses set forth typical valve steels which can bebenefited by the procedures of the present invention:

Once the melt is prepared, the next step consists in refining the grainsize of the casting. This can be accomplished by rapid solidification,vibration, by the use of electromagnetic fields, or by the addition ofsmall quantities of selected alloying elements. The latter procedure isthe most feasible commercially because of its simplicity. The additionof inoculants to the molten metal can produce grain refinements byrestricting grain growth through constitutionlal supercooling and/orproviding foreign nuclei for solidification of fine equiaxed grains.

To be effective, the inoculant should have the following properties:

I. stability in solid form in the molten matrix,

2. coherency with the matrix, i.e., crystallographic matching forwetting purposes,

3. similar density for dispersion, and

4. being sufficiently finely divided particles for distribution.

The addition of metals such as titanium, zirconium, vanadium, boron andmischmetal (a mixture of rare earth elements having atomic numbers of 57through 71 in metallic form, and containing about 50 percent cerium, theremainder being principally lanthanum and neodymium) have been found toprovide nuclei formers in the melt with. varying degrees of success. Byfar the most useful metal for this purpose is titanium and thesubsequent description of the preferred embodiments will be made inconjunction with that type of inoculant.

In accordance with the present invention, there is provided a cast slugof an austenitic stainless steel alloy inoculated with sufficientamounts of the grain refining additive to refine the grain size to avalue of 50 to 400 grains per square inch and a secondary dendrite armspacing of 0.02 mm. to 0.06 mm. to provide a slug having improved hotforging properties. The preferred grain size is in the range from 200 to350 grains per square inch. The measurement of the secondary dendritearm spacing is a measure of segregation and can be carried out on theslug castings according to the technique described by Wallace et al. inControl of Cast Grain Size," published by the US. Army Materials Agency,Watertown, Mass, as technical reference No. AMRA-CR 64-04/1964.

It appears that both constitutional :supercooling and independentnucleation are responsible for the grain refinement which occurs.Constitutional supercooling itself reduces the secondary arm spacing,but independent nucleation appears to be necessary for the production ofequiaxed grains.

The amount of inoculant can be varied, depending upon the pouringtemperature. An amount as small as 0.1 percent by weight is enough toconvert a columnar structure to an entirely equiaxed structure at apouring temperature of 3,000 F. At lower pouring temperatures, evensmaller amounts can be used.

A forged valve produced according to the present invention evidences agrain size of from 3 to 6 on the ASTM scale in the most heavily wroughtportions thereof, i.e., in the stem, and a coarser grain size of 50 to400 grains per square inch, and preferably from 200 to 350 grains persquare inch in the head portion thereof.

The following specific example illustrates the improvements in theresults obtained with the procedure of the present invention.

EXAMPLE Two identical alloy compositions were made up with a chemistryas set forth in Alloy C given above. One was solidified into a columnarstructure, and the other was treated with 0.13 percent titanium as aninoculant. The latter was produced in the following manner. The chargewas melted and brought to the pouring temperature of 3,040 F. The meltwas then deoxidized in the furnace by the addition of 0.25 percentaluminum. The melt was then slagged, followed by inoculation, stirringand pouring. The nontreated alloy slug was found to contain 75 grainsper square inch, while that which had been inoculated contained 340grains per square inch. Both slugs were tested at a temperature of 1,900F., with a crosshcad speed of 4.25 inches per second. The columnar,nontreated slug evidenced a reduction in area of 48.7 percent while thatwhich had been inoculated with titanium to produce an equiaxed structureevidenced a reduction in area of 54.3-percent. The breaking stress forthe unrefined material was 35,400 pounds per square inch, while that forthe refined material was 33,400 pounds per square inch. The reduction inbreaking stress and increase in reduction in area were indicative of animprovement in workability.

The actual processing of valves from these cast slugs confirmed theimprovements which had been achieved in the tensile tests. Exhaustvalves were extruded and coined at 2,100 from the two types of slugs. 1nthe case of the unrefined casting, external rupturing was quite severewhile in the case of the refined material, the rupturing was markedlyreduced.

The improvements in workability were obtained by titanium inoculation ofmedium carbon steels regardless of whether the casting was made withconventional sand castings or with directionally solidified castings asevident from the following table, all measurements being at 1,900 F. andat a erosshead speed of 4.25 inches per second.

We claim as our invention:

1. An exhaust valve for an internal combustion engine composed of awrought austenitic stainless steel alloy having a grain size of from 3to 6 on the ASTM scale in the stem portion and a coarser grain size of50 to 400 grains per square inch in the head portion thereof, said valvehaving titanium carbonitride compounds distributed therethrough.

2. The valve of claim 1 in which said coarser grain size is in the rangefrom 200 to 350 grains per square inch.

2. The valve of claim 1 in which said coarser grain size is in the rangefrom 200 to 350 grains per square inch.